Since the discovery of the use of metallic coatings on ferrous products as a means to deter corrosion of the underlying base; investigators have continuously sought to perfect improvements in coated products to prolong their life or to broaden their scope of application. Such attempts at improvement have followed many avenues. One of the most notable metallic coatings is zinc, exemplified by the widespread use of galvanized steel.
Galvanized steel is produced in a variety of conditions, namely unalloyed, partially alloyed or fully alloyed with the steel base, with a number of different surface finishes. All such varieties and/or finishes were the result of investigators seeking improvements in the coated product.
U.S. Pat. No. 2,110,893 to Sendzimir teaches a continuous galvanizing practice which is still followed today. The Sendzimir practice includes passing a steel strip through a high temperature oxidizing furnace to produce a thin film of oxide coating on the steel strip. The strip is then passed through a second furnace containing a reducing atmosphere which causes a reduction of the oxide coating on the surface of the steel strip and the formation of a tightly adherent impurity-free iron layer on the steel strip. The strip remains in the reducing atmosphere until it is immersed in a molten zinc bath maintained at a temperature of about 850.degree. F. (456.degree. C.). The strip is then air cooled, resulting in a bright spangled surface. The coating is characterized by a thin iron-zinc intermetallic layer between the steel base and a relatively thick overlay of free zinc. The thus coated product is formable, but presents a surface that is not suitable for painting due to the presence of spangles.
To produce a non-spangled surface which is readily paintable, a process known as galvannealing was developed. The processes described in U.S. Pat. No. 3,322,558 to Turner, and U.S. Pat. No. 3,056,694 to Mechler are representative of such a process. In the galvannealing process, the zinc coated strip is heated, just subsequent to immersion of the steel strip in the zinc coating bath, to above the melting temperature of zinc, i.e. about 790.degree. F. (421.degree. C.), to accelerate the reaction of zinc with the coating base steel. This results in the growth of the intermetallic layer from the steel base to the surface of the coating. Thus, a characteristic of galvannealed strip is a fully alloyed coating and the absence of spangles.
One area of interest that has garnered the attention of investigators was the need to improve the formability of the coated product. U.S. Pat. No. 3,297,499 to Mayhew, U.S. Pat. No. 3,111,435 to Graff et al and U.S. Pat. No. 3,028,269 to Beattie et al are each directed to improving the ductility of the steel base in a continuous galvanized steel. Mayhew's development subjects the galvanized strip to an in-line anneal at temperatures between about 600.degree. to 800.degree. F. (315.degree. to 427.degree. C.) followed by cooling and hot coiling. This treatment is intended to decrease the hardness of the steel base and increase its ductility without causing damage to the metal coating. The Graff and Beattie patents effect the same result with a box anneal treatment at temperatures between about 450.degree. to 850.degree. F. (232.degree. to 455.degree. C.). Finally, the same end result, i.e. improved steel base ductility, in this case for an aluminum clad steel base, is taught by U.S. Pat. No. 2,965,963 to Batz et al. The Batz et al. patent teaches heating an aluminum clad steel at temperatures in the range of 700.degree. to 1070.degree. F. (371.degree. to 577.degree. C.). Characteristic features of the processes of each of the preceding patents directed to post annealing of the coated product is to effect changes in the base steel without any recognizable metallurgical effect on the coating itself or on any improvements thereof.
The search for improved metallic coated products has not been limited to investigations of existing products. This was evidenced by the introduction of a new family of coated products, namely aluminum-zinc alloy coated steel, described, for example, in U.S. Pat. No. 3,343,930 to Borzillo et al, U.S. Pat. No. 3,393,089 to Borzillo et al, U.S. Pat. No. 3,782,909 to Cleary et al, and U.S. Pat. No. 4,053,663 to Caldwell et al. The inventions described in such patents, directed to aluminum-zinc alloy coated steel, represented a dramatic departure from past materials and practices, as the aluminum-zinc alloy coating is characterized by an intermetallic layer and an overlay having a two-phase rather than a single phase structure. Specifically, examination of the coating overlay revealed a matrix of cored aluminum-rich dendrites and zinc-rich interdendritic constituents.
Investigations have determined that such aluminum-zinc alloy coatings age-harden by as much as 35 VHN with an attendant loss in ductility. This age hardening is classic in the sense that it involves the precipitation of a second phase coherent with the matrix, which causes an increase in hardness and a decrease in the ductility of the coating. The present invention, as disclosed by these specifications, evolved as a result of the desire to improve the ductility of the coating, thereby broadening the usefulness of aluminum-zinc alloy coated ferrous products.